A common requirement for emergency marine safety equipment, such as life rafts and locator transmitters, is that it be released automatically if the vessel upon which it is carried should sink. Usually initiation of the release is designed to occur when a pre-determined depth (for example 4 meters) is exceeded.
Hydrostatic release mechanisms, which are commonly used to trigger the release of the safety equipment, react to the pressure increase that results when submersion to a specified depth occurs. The above release mechanism is itself actuated by the action of a hydrostatic pressure sensor. One such sensor uses an enclosed, air-filled chamber which has as one end a moveable diaphragm loaded by a compression spring from within. When submerged to a sufficient depth, the hydrostatic pressure against the outside of the diaphragm overcomes the combination of atmospheric pressure and spring force from the inside, and the diaphragm starts to move inward. The inward movement of the diaphragm can be used to initiate the release of the emergency equipment.
The foregoing method has the problem of serious triggering inaccuracy due to its not being independent of ambient air temperature or barometric pressure. This dependence occurs because the volume of air trapped within the chamber obeys normal gas laws and undergoes changes in absolute pressure in proportion to the absolute temperature, adding or subtracting to the force of the spring inside the chamber. In the case of changes in barometric pressure, an additional net force is also produced which adds or subtracts to the spring force (depending on whether the barometric pressure increases or decreases). Of the two mentioned sources of error, that of temperature dependence is by far the most serious, when such a device is expected to perform over the full environmental temperature range.
A known method of overcoming the above temperature related inaccuracy is the use of an aneroid as a pressure sensing device. This may take the form of a metal bellows, evacuated, and with an internal spring dimensioned to overcome the force due to atmospheric pressure and to provide an additional force equal to the force due to the hydrostatic pressure at the depth where triggering is desired. Since there is no gas within the aneroid, temperature dependency is eliminated, although the lesser dependence on barometric pressure remains. Although this method overcomes the difficulty of temperature dependence, a further problem arises due to the fact that absolute pressure is being measured, not the increase in pressure over atmospheric pressure. The hydrostatic pressure sensor must therefore be set to trigger at a pressure equal to the sum of the hydrostatic pressure at the desired release depth and atmospheric pressure, instead of the hydrostatic pressure alone. Since the trigger points desired are usually less than the equivalent of 5 meters of water head, the accuracy of the triggering point is much reduced since the hydrostatic pressure is only a small fraction of the total pressure being measured.
A known means of overcoming the sensing errors due to ambient air temperature and barometric pressure changes when using an air filled chamber with a moveable spring loaded diaphragm as the sensing device is to provide a small orifice into the chamber or to mount a small section of porous material over an opening into the chamber. The material acts as an orifice which provides greatly restricted air flow from inside to outside the chamber. By this means, air pressure inside the chamber will always reach an equilibrium with that outside, as slow changes in temperature and barometric pressure occur. Because of the small size of the orifice or the limited porosity of the porous material, if the latter is used, the chamber will respond to rapid changes in pressure such as reasonably quick immersion to a required triggering depth in water, providing that the application of this pressure occurs much faster than the orifice will allow equilibrium to occur. A significant disadvantage of this method is that the pressure sensing chamber is no longer completely sealed against the entry of liquid water. This causes two problems. First, the depth trigger point is affected if slow rates of immersion occur caused, for example, when slow submersion occurs due to water entry into the pressure chamber. Second, water entry into the pressure chamber may occur when the device is stored for long periods and exposed to rain and water spray in its normal shipboard mounting position. Such water entry interferes with operation of the device.
Accordingly, it is an object of the invention to provide an improved liquid pressure sensor. It is yet a further object of the invention to provide a hydrostatic pressure sensor which accurately compensates for changes in ambient temperature and barometric pressure.